Scanning probe microscopy techniques provide imaging, force measurement, and molecular manipulations. Scanning probe microscopy techniques provide higher resolution than the optical diffraction limit through use of a sharp scanning probe that contacts a surface. Types of scanning probe microscopy include atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning near-field optical microscopy (SNOM), and tuning fork scanning probe microscopy.
Tip-enhanced Raman spectroscopy (TERS) combines Raman spectroscopy that leverages an atomic force microscope. TERS can also be performed with an STM or in tuning fork feedback operational mode. TERS is a powerful optical technique for resolving features of a surface. It is of particular interest for chemical mapping, and provides the potential to achieve quantitative spectroscopic analysis of arbitrary surfaces with nanoscale resolution. TERS combines ultrasensitive vibrational spectroscopy with scanning probe methods by using a nanoscale probe tip to probe a surface. Unfortunately, typical manufacturing techniques for batch manufacturing of TERS probes yield high variability from probe tip to probe tip.
TERS at present is primarily a research technique because batch manufacturing of precise tips has not been provided. Research groups with extensive plasmonic and fabrication knowledge can fabricate individual tips effectively enough to perform TERS experiments. The commercial tips currently on market are expensive and unreliable. This prevents companies and research groups from investing in TERS equipment. If the tips could be be fabricated and sold with a low cost and high reliability, the technique could become more prevalent and routine, growing the market both for TERS equipment and TERS tips. There are several markets interested in TERS seeing commercial reality, the largest of which is the semiconductor industry. TERS can give information about dopants, stress/strain, material composition, crystal orientation, and other properties of semiconductor and solid state devices that are unobservable with other current techniques. There are many interesting biological questions that can potentially be addressed with TERS as well. There are also biological sensors and characterization applications for TERS, for example, it has been demonstrated that TERS could be used to sequence DNA. See, e.g., Kolodziejski, N., “Tip-enhanced Raman spectroscopy for the base interrogation of DNA,” Methods Cell Biol., 114:611-28 (2013). It may also be useful for looking at cell membrane composition, and in many other biological applications. See, e.g., Kumar et al., “Tip-enhanced Raman spectroscopy: principles and applications,” EPJ Techniques and Instrumentation 20152:9 (Jul. 1, 2015).
The primary methods of fabricating AFM probe tips for TERS include electrochemical etching of metal wires, evaporation of metal onto probe tips, and top down fabrication. See, e.g., Kharintsev, S. S.; Hoffmann, G. G.; Fishman, A. I.; Salakhov, M. K. Plasmonic Optical Antenna Design for Performing Tip-Enhanced Raman Spectroscopy and Microscopy. J. Phys. D: Appl. Phys. 2013, 46, 145501; Taguchi, A.; Yu, J.; Verma, P.; Kawata, S. Optical Antennas with Multiple Plasmonic Nanoparticles for Tip-Enhanced Raman Microscopy. Nanoscale 2015, 7, 17424-17433; De Angelis, F.; Das, G.; Candeloro, P.; Patrini, M.; Galli, M.; Bek, A.; Lazzarino, M.; Maksymov, I.; Liberale, C.; Andreani, L. C.; Di Fabrizio, E. Nanoscale Chemical Mapping Using Three-Dimensional Adiabatic Compression of Surface Plasmon Polaritons. Nat. Nanotechnol. 2010, 5, 67-72. Electrochemical etching of metal wires involves a gold or silver wire that is inserted through a gold/platinum circular electrode and submerged in an acidic solution. The wire is etched when a bias is applied. When thin enough, the bottom part of the wire breaks away, leaving a sharp tip at the end which functions as a TERS tip. Evaporation of metal onto AFM probes coats commercial AFM probes with an evaporated film of metal 40-70 nm thick. The film at the apex of the tip acts as a nanoparticle. This is one of few processes that can be used as a batch fabrication technique.
There are a few examples where groups have been able to get the film to anneal into islands, creating separate nanoparticles that improve on TERS quality, they still suffer from the same problems of unpredictability, unreliability however. Top down fabrication includes a variety of engineered TERS tips that use extremely low throughput and expensive machines (focused-ion beam and electron beam) to fabricate highly precise tips. These tips are high performance but can take dozens of hours for highly trained individuals to fabricate.
Johnson et al., report fabrication of (10 nm) metallic probes suitable for scanning probe microscopy and spectroscopy techniques. See, Johnson et al., “Highly Reproducible Near-Field Optical Imaging with Sub-20-nm Resolution Based on Template-Stripped Gold Pyramids,” ACS Nano, Vol. 6, No. 10, pp 9168-74 (2012). This fabrication technique deposits gold into a shaped template and strips the formed gold tips out of the template. A silicon nitride mask is used to define regions for anisotropic etching of silicon with KOH. This patterns a high quality silicon wafer with pyramid shaped hollows having an apex angle of 70.52°. Gold is deposited into the hollows. A lift-off frees the gold deposits leaving isolated gold pyramids, which can then be stripped out of the hollows using epoxy and a thin tungsten wire. The pyramids are attached to the tips via the epoxy that strips them from the mold. The tips in this case are not AFM tips, but are instead tuning fork tips. The tips are formed from wires, which are epoxied or otherwise attached to a small quartz crystal that turns the whole tip into a resonator. The tuning fork tips can be in a scanning probe feedback mode that is similar to the use of AFM probes and techniques.
Scanning tunneling microscopy (STM) relies upon quantum tunneling between a surface and an STM tip. The STM tip is a conductive tip is brought close to a surface and a bias between tip and the surface electrons to tunnel through the vacuum between them. STM tips used for TERS are silver or gold, which are the plasmonically active materials.